How The Bacteria In Our Mouths Cause Tooth Decay (And How to Prevent It)

This blog post was originally published on September 20, 2019 and updated to include a newly-recorded podcast episode on June 2, 2021.

When you think of your dental cleanings, I bet you’re like, “Oh, goodie! I can’t wait to scrape those darn bacteria off my teeth to prevent tooth decay!” 

Just kidding. But hopefully, it’s not how I felt growing up.

As a kid, I hated going to the dentist. I remember saying I would rather die than go to the dentist. Dramatic. I know. My fear subsided when I was ~16 and had a root canal due to a childhood injury. (I went over the handlebars on my bike and ate pavement). After the root canal, I realized the dentist wasn’t as terrible a place as I had initially thought.

Soon after the root canal, I began working at a dentist’s office, which was strange after having such an intense fear as a kid, but it continued to help alleviate it. A big part of my anxiety stemmed from the fear of the unknown. So, working at the dentist’s office gave me the chance to see the worst-case scenarios (of general dentistry at least), and well, I could handle it. 

But this is not about dental fears — it’s about the bacteria that cause dental caries (aka cavities or tooth decay). These bacteria are the main reason why we need to go to the dentist (and clean our teeth).

Here, I’ll tell you about the bacteria that live in our mouths, how they cause cavities, and how our dental care helps prevent tooth decay.

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The oral microbiota: a microbial ecosystem in the mouth

The mouth is an ecosystem full of bacteria, fungi, and viruses, and protozoa. Although this ecosystem contains many different microbes, I’ll be focusing on bacteria here. The human mouth is home to hundreds of bacterial species. Over 700 species have been identified in humans and have been cataloged in the human oral microbiome database.

We have some bacteria that guard our mouths, protecting us from pathogens. These residents are called commensal bacteria, and we don’t typically worry about them. We want them there. But how do they provide protection? They live on our teeth or occupy the niche where the pathogens would like to live. And they are experts at inhabiting this niche and keeping invaders out, a property called colonization resistance.

Some pathogens live in our mouths too, unfortunately, which is how we end up with dental issues. These pathogens are present in health but at low levels compared to diseased teeth. But how do the pathogens usually remain in check? Our commensal bacteria, our immune system, and eating a healthy diet provide protection and help keep pathogens at bay. 

What we eat affects the bacteria in our mouths and tooth decay

What we eat affects the bacteria in our mouths. When we eat a lot of sugar, the mouth environment changes, and the commensals can lose the battle against the pathogens. 

Bacteria in our mouths can ferment the sugars we eat, leading to the production of acid. This process is the same as what happens in food fermentation. The acid the bacteria produce can wear away the protective enamel on our teeth. 

A highly acidic mouth environment favors pathogens that can tolerate acid. As plaque, which is a biofilm (bacteria suspended in a matrix), builds up, our saliva, which normally neutralizes acids, cannot flow as well. As a result, the biofilm environment becomes highly acidic, and tooth decay can begin. (If you’d like to read more about this process, check out my friend Sarah’s blog post on dental caries, which also has some great visuals.)

Originally it was thought that dental caries was exclusively caused by Streptococcus mutans. Now, after sequencing the oral microbiome and specifically looking at the microbes associated with dental caries, it appears that more bacteria are involved. Ultimately, dental caries is associated with a shift from commensal-dominated to pathogen-dominated communities. This shift is called dysbiosis.

How does brushing your teeth help prevent cavities?

Brushing your teeth physically removes plaque, the biofilm on your teeth. And we need to remove that plaque regularly and prevent it from building up. The thicker the biofilm, the less access saliva has to be able to neutralize the acids produced by the bacteria. 

So, we need to reduce the thickness of the biofilm that has built up. If not, the environment in the biofilm (plaque) becomes more and more acidic, and the acid-tolerant and acid-producing pathogens thrive. The abundance of acid wears away enamel and outpaces the buffering abilities of saliva.

What about the commensals? Do we really want to remove them too? Although you are removing both pathogens and commensals, you want to keep that biofilm in check to prevent the plaque from becoming a highly acidic environment.

Bushing our teeth with toothpaste that contains fluoride provides a benefit to our teeth. Fluoride helps restore the enamel, which strengthens your teeth. According to the American Dental Association, fluoride in our toothpaste is not only safe but also effectively prevents tooth decay. Here are two interviews with dentists, and they recommended using fluoride-containing toothpaste (interview 1 and interview 2).

What do dental cleanings do to help prevent tooth decay?

When we go to the dentist, they scrape away the buildup of hardened plaque, which is called calculus or tartar. We cannot remove it simply by brushing our teeth with a toothbrush. So, our dental cleanings remove that tartar.

If we don’t remove the tartar, it provides more areas for plaque to build up. These areas, called fissures, are like cracks in rocks and are crevices where bacteria can form more plaque.

So, bacteria play both positive and negative roles in our dental health, but we ultimately need to keep our teeth clean to prevent cavities. Below are some practical tips that will help you maintain your dental health.

Practical Tips For Dental Health

• Avoid sugar
• Avoid acidic foods
• If you eat lots of sugary or acidic foods, rinse your mouth with water afterward
• Brush with fluoride toothpaste
• Get your teeth cleaned regularly at the dentist